Vanadium-containing tool steel article

ABSTRACT

A VANADIUM-CONTAINING TOOL STEEL AND ARTICLE COMPACTED FROM PARTICLES THEREOF AND A METHOD FOR PRODUCING SAID ARTICLE FROM A PREALLOYED POWDERED CHARGE OF THE ALLOY CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, CARBON 1.5 TO 1.6, CHROMIUM 3.5 TO 4.5, VANADIUM 2.5 TO 3.5 TUNGSTEN 9 TO 11, MOLYBDEMUM 4.5 TO 5.5, COBALT 8.0 TO 12.5, MANGANESE UP TO 1, SILICON UP TO 1, NITROGEN .02 TO .08 AND BALANCE IRON; THE ARTICLE IS CHARACTERIZED BY THE COMBINATION O HARDNESS AND TOUGHNESS MAKING IT DESIRABLE FOR CUTTING TOOL APPLICATIONS.

May 7, 1974 G. STEVEN VANADIUM-CONTAINING TOOL STEEL ARTICLE 2 Sheets-Sheet 1 Filed Oct. 24, 1972 a w 2 g m VII/ll; M F c N w m 0 26% @5323 33 5% ms #8 kuqmaq w 5 0 T 5 4 3 0 5' M 4 8 0 m M MW W 0 0 O 0 N a 6 4 a 0 W B wsimi m m wwQGmwu mat -u b6 SSQSQ c m F/GI y 1974 G. STEVEN 3,809,541

VANADIUM-CONTAIMNG TOOL STEEL ARTICLE Filed 001;. 24, 1972 2 Sheets-Sheet B United States Patent Office 3,809,541 Patented May 7, 1974 3,809,541 VANADIUM-CONTAINING TOOL STEEL ARTICLE Gary Steven, 444 Royce Ave., Pittsburgh, Pa. 15216 Filed Oct. 24, 1972, Ser. No. 300,094 Int. Cl. C22c 39/14 U.S. Cl. 29-192 2 Claims ABSTRACT OF THE DISCLOSURE In the use of tool steel articles for high-speed cutting tool applications, a good combination of wear resistance and high toughness is necessary for obtaining prolonged tool life. Wear resistance is promoted by high hardness and, particularly, by high MC-type (vanadium) carbide content of the tool steel. On the other hand, toughnesscommonly measured in terms of impact strengthalso contributes to tool life in that, particularly in intermittent cutting applications, the tool is able to withstand repeated impacts during cutting without breaking. But the combination of high wear resistance and high toughness is diflicult to achieve, because of the MC-type carbide content is increased by the necessary changes in the composition of the steel, the toughness or impact resistance of the steel decreases.

It is accordingly the primary object of this invention to FIG. 1 is a graph showing the amount of MC-type (vanadium) carbide in the as-hardened structure of conventional high-speed steels AISI M1, M4 and T15;

FIG. 2 is a similar graph showing the efiect of MC-type (vanadium) carbide contents on the toughness of these three conventional steels when heat treated to a hardness of R 65;

FIG. 3 is a graph showing the effect of the MC-type (vanadium) carbide contents of these three conventional steels on the wear resistance when heat treated to a hardness of R 64;

.FIG. 4 is a photomicrograph of a steel compact in accordance with the present'invention showing the carbide size and distribution thereof at a magnification of 1000 and FIG. 5 is a photomicrograph showing the prior austenite grain size of the compact of FIG. 4 at a magnification of 1000X.

The tool steel of the invention consists essentially of, in weight percent, carbon 1.5 to 1.6, chromium 3.5 to 4.5, vanadium 2.5 to 3.5, tungsten 9 to 11, molybdenum 4.5 to 5.5, cobalt 8.0 to 12.5, manganese up to 1, silicon up to 1, nitrogen .02 to .08 and balance iron. In accordance with the invention, this steel is used in the form of powder of about minus 16 mesh US. Standard. This powder is placed in a metal container, which is gas tight.

The container is heated to an elevated temperature in excess of about 2000 F. and during the initial stages of heating its interior is pumped to a low pressure whereupon the gaseous reaction products and principally those resulting from the reaction of carbon and oxygen are removed from the interior of the container, which operation is termed as outgassing. Thereafter and upon rernoval of the gaseous reaction products, the container is heated to or above the desired compacting temperature,

which is in excess of about 2130 F., and with the container sealed against the atmosphere it is transferred to a compacting apparatus. Compacting may be achieved by use of a mechanical apparatus wherein the sealed container is placed in a die and a ram is inserted therein? to compact the container and charge. Alternately,' 'the container may be placed in a fluid-pressure vessel, 'corn 1900 F. If desired, compacting to this density' may be i achieved by a plurality of separate compacting" opera-'- tions, and the powder charge may be precompacted to a low density, e.g. 60%, prior to outgassing. After com' pacting, conventional machining or forming operations incident to cutting tool manufacture may be performed.

It was heretofore believed, as may be seen from a study of FIGS. 1, 2 and 3 of the drawings, that as the vanadium content and correspondingly the carbon content of the alloys of this type are increased to increase the MC-type (vanadium) carbide content of the alloy its wear resistance is increased while its toughness, as a function of impact value, is decreased. With the present invention it has been found, however, that if carbon is maintained'within the range of 1.5 to 1.6% in combination with a vanadium content of 2.5 to 3.5%, as will be specifically explained and shown hereinafter, high Wear resistance may be achieved without an attendant decrease intoughness. m

In the heat treating of articles in accordance with the invention, the articles are austenitized at a temperature on the order of 2170 F. and then hardened during cooling. The austenitizing step involves heating to a temperature sufiicient to dissolve, to a considerable extent, the.

carbide phases present in the microstructure of the steel. After quenching from the austenitizing temperature, the article is subjected to reheating at a lower temperature in which case carbide-forming elements, e.g. vanadium, tungsten ,and molybdenum, are precipitated in the form of fine carbides. This, of course, produces the secondary hardening necessary for high-speed cutting applications. During austenitizing, much of the carbon is dissolved in the 'austenite, which upon cooling transforms to a hard carbon-containing martensite. The carbide-forming elements remain in solution in the martensite. Subsequently, however, the carbide-forming elements during tempering combine with the carbon in the steel and form'carbides. It is this carbide precipitation which produces the desired secondary hardening.

As above stated, the restricted combination of carbon and vanadium in the alloy achieves the result of a good combination of wear resistance and toughness. In addition, the cobalt present in the alloy contributes to the retention of hardness at high temperature, to which the alloy articles are subjected during high-speed cutting use. Nitrogen is necessary to achieve a desired fine carbide distribution. Both molybdenum and tungsten within the recited limits are necessary to obtain suflicient heat resistance for operation of a cutting tool at high speeds.

TABLE I Composition, weight percent Steel des- Internal ignation code Mn S P Si Cr V W Mo Co N To demonstrate the present invention, and by way of specific examples thereof, samples of the steels designated as A, B, C and D with the compositions listed in Table I were produced. Steel B is within the composition limits of the present invention; whereas, steels A and D have carbon contents below that of the steel in accordance with the present invention, and steel C has a carbon content higher than that of the steel of the present invention.

More specifically, powdered charges of steels A, B, C and D of Table I were made by conventional gas atomizing and screened to a size consist of minus 16 mesh US. Standard. Each of the particle charges of steels A, B, C and D were processed in the following manner. A particle charge of A and B was placed in a mild-steel cylinder about 6" length and having a diameter of 5 /2". The particle charge of C and D was filled into a cylinder about 48" length and having a diameter of about 11". The particle-filled container was heated to a temperature of about 2100 F. for about 5 hours, and during the early stages of heating the interior of the container was connected to a vacuum pump which was used to remove the gaseous reaction products from the container. The container and charge were compacted to achieve a density of about 99% of theoretical by the use of an isostatic compaction pressure of 15,000 p.s.i. in a gas pressure vessel with nitrogen being the gaseous pressure medium employed.

After compacting, the compacted article was forged into 1" square bars. The bars were then austenitized at 2160" F. for 2 minutes and then oil quenched. Microscopic examination of these bars showed a uniform dispersion of carbides in an iron-rich alloy matrix, and further that the carbide phase particles were predominantly less than 3 microns in size.

To test the tool performance, /2" square tool bits were prepared from forged bars of steels A, B, C and D; the tool geometry was: 3, 6, 10, 10, 10, 10, 0.030" nose radius. These tool bits were heat treated by austenitizing at 2160 F. for 2 minutes, quenching into a salt bath at 1100 F., holding for 5 minutes, air cooling to ambient temperature, and tempering at 1025 F. for 2+2+2 hours. The resulting hardnesses are included in Table II. The microstructure of steel B is shown in FIGS. 4 and 5.

For comparison, /2" square tool bits with the same geometry as the tools produced from the bars of steels A, B, C and D were prepared from commercial stock of the well-known AISI Type T high-performance highspeed tool steel and heat treated in accordance with the recommended commercial practice to 67 R hardness. First, all tool bits were tested for performance in continuous lathe turning of an AISI H13 workpiece heat treated to 300 BHN. The cutting speed was 55 s.f.p.m.; feed0.010" per revolution; depth-of-cut ;i ";c0olant none; test end-point 0.015" flank wear. The results of Table II show that, as expected, tool life was increased with increased vanadium carbide content and hardness. Steels A, B, and C as a group were distinctly superior to steel D as well as to AISI Type T15 high-performance high-speed tool steel. Within the A, B, C group, tool life increased from steel A to steel B to steel C, i.e., in the order of the increasing carbon or vanadium carbide contents, as expected.

Then intermittent cut-lathe turning tests were conducted using a four-slotted AISI H13 workpiece heat treated to 300 BHN. The speed was 50 s.f .p.m.; feed 0.010" per revolution; depth-of-c'ut coolantnone; test end-point 0.015" flank wear. At least three tests were run on each tool material. The results of Table II revealed an unexpected resultthe intermittent-cut tool life of steel B (containing 1.56% carbon) was distinctly superior to those obtained with steels A (lower carbon) and C (higher carbon) as well as steel D (lower ca'rbon and lower vanadium) and AISI Type T15, which is well known as a high-performance high-speed steel. A definite criticality of the carbon content of the inventive steel in the range of 1.50 to 1.60% is indicated. This finding is even more surprising when the higher hardness of steel B (R 69.5) in comparison to steel A (R, 68.5) and steel D (R 68.5) is taken into consideration. As explained hereinabove, at this higher hardness, one would expect that the toughness of the tool bit of steel B, as demonstrated by intermittent cut tool life, would decrease.

What is claimed is:

1. A full dense, tool-steel, powder-metallurgy compacted article produced from prealloyed tool-steel powder consisting essentially of, in weight percent, carbon 1.5 to 1.6, chromium 3.5 to 4.5, vanadium 2.5 to 3.5, tungsten 9 to 11, molybdenum 4.5 to 5.5, cobalt 8.0 to 12.5, manganese up to 1, silicon up to 1, nitrogen .02 to .08 and balance iron.

2. The article of claim 1 having a substantially uniform carbide dispersion with the carbide particles predominantly less than three microns in size.

References Cited UNITED STATES PATENTS 1,206,834 12/1916 Furness l26 E 1,876,725 9/1932 Mitchell 75l26 E 1,998,956 4/1935 Emmons 75l26 E 2,590,835 4/1952 Kirkby 75126 E HYLAND BIZOT, Primary Examiner US. Cl, X.R.'

75l26 C, 126 V, 126 H, 126 A 

